1. Field of the Invention
The present invention is directed to drag bits having diamond or other hard cutter elements. More particularly, the present invention is directed to blade-type drag bits incorporating multiple diamond disc cutters. As the blades erode during drilling in a formation, new diamond elements are continuously exposed for attacking the formation.
2. Brief Description of the Prior Art
Drill bits or rock bits are well known in the art. Such drill bits are used for drilling in subterranean formations when prospecting for oil, water or minerals. The term "drag bit", generally speaking, designates a drill bit which has no rotating cones and which is rotated either from the surface through a string of drill pipes and drill collars (drill string) or by a suitable "downhole" motor. In contrast, rotary cone bits have one or more journals each of which carry a freely rotatable drill bit cone. Regardless of whether rotary cone bits or drag bits are used for drilling in a formation, drilling fluid or "drilling mud" is continuously circulated from the surface through the drill string down to the drill bit, and up to the surface again. As is well known, the circulating mud serves several important functions; these include continuous cooling of the drill bit and removal of the downhole cuttings which are generated by the drilling action.
Several types of drag bits are known in the art; these include fishtail bits, auger bits, as well as more "conventional" drag bits which lack relatively large extending blades but nevertheless may be provided with "hard" diamond, tungsten-carbide, or the like cutter inserts. Blade-type rotary drag bits are also known in the art which have diamond or other "hard" cutter inserts imbedded or affixed to the blades. Such blade-type bits are described, for example, in U.S. Pat. Nos. 4,440,247, 4,499,958 and the aforementioned U.S. Pat. No. 4,813,500.
Generally speaking, one serious problem encountered in the prior art in connection with diamond insert studded drag bits is overheating of the diamond inserts due to inadequate flushing and cooling action of the drilling fluid. As is known, heat, unless dissipated through adequate cooling with drilling fluid, may convert the diamond of the inserts into graphite with a resulting loss of hardness and drilling power. Thereby allowing the inserts to rapidly wear away.
The prior art has attempted to solve the foregoing problems by providing drilling fluid outlet passages or holes adjacent to the diamond inserts in the drag bits, and by appropriately choosing the configuration of the drag bit body so as to optimize the flushing and cooling action of the drilling fluid on the cutter inserts. The drill bits described in U.S. Pat. Nos. 4,221,270, 4,234,048, 4,246,977, 4,253,533, 4,303,136, 4,325,439, 4,334,585, 4,505,342, and 4,533,004 provide examples of these efforts in the prior art.
Still further description of drill bits, which comprise a general background to the present invention, may be found in U.S. Pat. Nos. 3,938,599, 4,265,324, 4,350,215, 4,475,606, 4,494,618, 4,538,690, 4,538,691, and 4,539,018. A general overview of "Rock-Bit Design, Selection, and Evaluation" may be found in a paper bearing the above title. This paper is a revised reprint of a presentation made by H. G. Bentson at the Spring meeting of the Pacific Coast District, API Division of Production, Los Angeles, May, 1956, printed in August, 1966.
A problem associated with fishtail type bits is to maximize the cutting efficiencies of each blade of the bit. Generally speaking, conventional blade bits provide a blade leading edge angle of attack relative to the bottom of a borehole that is perpendicular to the bottom of the borehole.
The present invention teaches rows of strategically positioned tungsten carbide backed diamond discs mounted to the cutting face of a blade, each cutting disc having a negative angle of attack with respect to the borehole bottom, affording heel clearance, thereby generating less heat, because of the preferential wear of the softer tungsten carbide in relation to the diamond table, therefore greatly extending the wear life of the diamond disc cutters.
In summary, the foregoing patent disclosures provide evidence of intense efforts in the prior art to develop rock bits in general, and diamond cutter insert studded drag bits in particular, which have prolonged working lives and improved wear characteristics. In spite of the foregoing efforts, there is definitely need for improvement in this field. Specifically, there is a need in the art for blade-type drag bits having diamond cutter elements retained on the blade with negative cutter element attack angles with respect to a borehole bottom and for said cutters to be positioned in radial rows with each row leaving kerfs of formation to be destroyed by each succeeding row of cutters axially positioned to completely over-lap these uncut kerfs thereby greatly increasing the drilling rates because by definition Poisson's ratio show the shear strength of the unsupported kerfs cannot be greater than 50% of the compressive strength of the totally supported virgin rock.